When carrying out a chemical reaction, be it an industrial process or a research experiment, one starts with a number of reactants and, in some cases, obtains a mixture of various products with or without traces of the starting reactants. Usually, this requires tedious separation of each constituent of the mixture yielded and only then identification of each product obtained.
NMR spectroscopy is an excellent method for examining the molecular structure of pure compounds. However, it is limited when examining mixtures of compounds. To a certain extent 2D and multidimensional NMR can separate simple mixtures but a more suitable method is to separate the components according to their diffusion coefficients.
The diffusion ordered spectroscopy (DOSY) method (Morris et al, J. Am. Chem. Soc., 115, 4291-4299 (1993)) can separate compounds in a mixture and with the correct processing can ideally yield regular 1D-spectra for each compound in the mixture (Schilling et al. GIT Labor-Fachzeitschrift, 46, 1163. (2002)). One of the problems associated with this method is that in most cases there is insufficient separation in the diffusion axis to fully separate the compounds in the mixture.
An attempt to enhance the separation in the diffusion dimension was made by adding a solid “chromatographic” medium (as used in conventional liquid chromatography) such as silica gel or modified silica gel to the sample. However, for conventional high-resolution NMR, this causes excessive line-broadening precluding any diffusion measurement or useful spectral resolution. The use of magic-angle spinning under HR-MAS conditions (FR2847674 and Pages et al. Anal. Chem., 78, 561-566 (2006)) or the use of other techniques like matching the magnetic susceptibility of a solution to the magnetic susceptibility of the silica (WO 2008/050347, Hoffman et al, J. Magn. Reson 194, 295-299 (2008)) were suggested for obtaining useful resolved spectra. However, HR-MAS required significant modification to the spectrometer, and also does not guarantee the sample homogeneity necessary when using solid particular matter in the examined sample. Additionally, the rotation causes the diffusion measured to deviate from its non-spinning value.
The conditions required when using susceptibility matching techniques make this method limited in practice. Such conditions include, for example, the use of photosensitive brominated or iodated solvents (due to the high diamagnetic susceptibility of silica gel compared with regular solvents); in addition samples comprising solid silica need instant immediate preparation and constant mixing of said sample prior to the experiment (due to the fact that silica tends to settle or float in solution). In practice, there remains significant broadening of the signals by several Hertz in silica based examples. Additionally, since magnetic susceptibility of a solution is temperature dependent, this ultimately precludes performing DOSY experiments at variable temperatures. All these limitations restrict the range of systems for which silica can be used to separate component mixtures.
There is therefore a need for a liquid medium capable of affording a resolved diffusion ordered spectra of a sample comprising a mixture of compounds that is both stable and less susceptible to magnetic susceptibility effects and limitations.